A prosthetic implant system with a device for measuring the stress/strain relationship in the body of the implant.
Surgically implanted prosthetic devices provide remedy for physical damage due to disease, injury, surgery, and other events. One type of prosthetic implant fulfills its role by filling surgical voids, as in breast cancer surgery. Another type of prosthetic implant carries out a simple structural role, such as replacing sections of bone. A further type of structural prosthetic implant articulates and replaces the function of a working joint such as a knee, hip, or other body joint. In all of these cases the implant takes up substantial space in the body that can be used for one or more additional functions.
By way of illustration, during hip-replacement surgery, the stem of the replacement joint is forced into a mating cavity that has been reamed into the femur; likewise the socket portion of the implant is attached to a mating cavity in the pelvis. The preferred method of implanting the stem does not use adhesive or other filling materials, so that the native bone can grow into intimate attachment with the external surface of the implant stem. Following surgery the patient is physically restricted, in order to provide a very stable environment for initial bone growth and attachment. Some time later, initial light movement is permitted, and at a later time, modest exercise is begun. However, there is no way to directly measure the onset of bone growth at the interface between the implant stem and femur. The variability in onset and progress of bone growth depends upon a wide range of variables including the patient""s age, health, and gender, the specific surgical procedure, previous joint stress and damage, drug therapies for this and other maladies, uptake and metabolic conversion of calcium and phosphorous, and other factors. Thus the existing method and postoperative regimen for hip replacement result in the use of a conservative and normative approach to exercise, and provide little or no information relating to the success of the graft. In one case, a patient needing further therapy, including the application of drugs or other therapeutic effects, will not receive it. In the other case a patient is restricted from physical activity for a longer period than necessary, causing inconvenience and unnecessary financial cost.
Current methods for direct measurement of the progress of bone healing associated with joint implant surgery are limited in their ability to provide useful information needed to optimize post-operative care. Thus, e.g., U.S. Pat. No. 4,922,892 discloses a device and method for determining proper seating of a hip prosthesis by testing the friction fit of said prosthetic hip implant by applying a torsional force for a period of seconds on said prosthetic hip implant, with the fit determined to be correct if said prosthetic hip implant does not experience rotational micromovement, thus using a coarse physical stress applied at the time of surgery to determine proper seating. U.S. Pat. No. 6,245,109 discloses a system that includes an artificial joint implantable within an individual, the artificial joint including artificial joint members each having an articulating surface and a bone attachment surface, wherein the articulating surfaces are in articulative engagement therebetween. The system further includes a detection system implanted within, or attached to, the artificial joint members and/or the bones to which the members are attached. U.S. Pat. No. 6,245,109 is intended to monitor appropriate range-of-motion of the fully healed joint implant and warn of the onset of joint failure at a later time; as such it does not provide for information or therapy relating to the healing process between implant and bone. U.S. Pat. No. 5,326,363 discloses a provisional joint component having a functional surface of a generally nonconductive material interspersed with conductive regions; an electrical circuit is established when a conductive portion of a mating component contacts one or more of the conductive regions on the provisional component; as such it also provides a very coarse measure of relative movement between surfaces, and does so in a manner that may result in electrolytic damage to local tissues. U.S. Pat. No. 5,456,724 discloses a load sensor inserted between two bones for sensing a stress exerted on the load sensor includes a body, and a load transducer mounted in the body for transducing a strain thereof resulting from the stress into an electronic signal; such load sensor can detect a stress exerted on a bone graft bone and/or the interaction of the bone graft and spine implant to improve curative effects and increase the successful rate of a bone graft operation; as such the invention of U.S. Pat. No. 5,456,724 is useful in some applications of bone graft surgery, but due to its method of measurement of tension or compression across a graft joint, it would be very insensitive to the increased strength associated with proper healing of a joint implant.
By way of further illustration, U.S. Pat. No. 6,034,296 discloses an implantable self-powered sensing system for a bone fixation device, which includes a self-powered strain sensor mountable on or in a bone fixation device; the sensor is capable of measuring strain in the bone fixation device by generating a strain signal in response to stresses produced thereupon, and the system also includes a telemetry unit powered by the generated strain signal and in communication with the sensor. The device of this patent monitors healing in a bone by measuring strain in a bone fixation device using the above and by subjecting the patient to a predetermined set of dynamic exercises which vary stresses exerted on the bone fixation device and tests the integrity of the bone fixation device; the charge in response to the stresses imposed on the bone fixation device then powers the implanted telemetry unit in order to transmit the signal generated by the sensor unit to an external receiver as a corresponding strain measurement signal. While such device provides some measure of the degree of healing, its use is focused on fixation devices that carry the mechanical strain and, as with the other referenced inventions, does not provide a good indication of the progress of implant integration into native bone.
The devices and processes of the patents discussed in this section, the entire disclosure of each of which is hereby incorporated by reference into this specification, may be used to monitor the bone healing progress after joint implant surgery; but they are not effective in indicating the optimal time for ehancement of bone regrowth.
Current methods for enhancement of bone regrowth after prosthetic implant surgery involve supplying various chemical and/or drug compounds to the growth site, as well as providing electrical stimulation for the migration of calcium and other materials directly to the growth site. Thus, e.g., reference may be had to U.S. Pat. No. 5,383,935, which discloses a prosthetic implant for implantation into skeletal bone comprising an implantable base member having an internal housing surrounded partially by a porous wall, an anode secured in the internal housing in electrical contact with the porous wall, and a porous means placed in the internal housing for retaining a biocompatible electrolyte. Thus, e.g., U.S. Pat. No. 6,121,172 discloses composite materials formed from bioactive glass or ceramic fibers and structural fibers, used to enhance bone regrowth at the interface between native bone and an implant. U.S. Pat. No. 6,143,035 discloses a system for enhanced bone healing or, anchoring of an implanted bone prosthesis such as a plate, stem, articulation component or other structural component, utilizing a piezoelectric element that is coupled to receive mechanical strain from body activity and that generates a charge which is applied to enhance bone growth for anchoring the prosthesis. U.S. Pat. No. 6,120,502 discloses an electrical bone growth promotion apparatus and method for the delivery of electrical current to an implant surgically implanted within the intervertebral space between two adjacent vertebrae of the spine to promote bone growth and the fusion process to areas adjacent to the implant. U.S. Pat. No. 6,034,295 discloses an implantable device with a biocompatible body having at least one interior cavity that communicates through at least one opening with the surroundings of the body so that tissue surrounding the implantable device can grow through the opening; two or more electrodes within said having terminals for supplying a low-frequency electrical alternating voltage and at least one of which is located inside the cavity. U.S. Pat. No. 5,030,236 also discloses the use of electrical energy, but unlike the other references above, it relies upon radio frequency energy coupled inductively into an implanted coil to provide therapeutic energy. U.S. Pat. Nos. 5,383,935, 6,121,172, 6,143,035, 6,120,502, 6,034,295, and. 5,030,236 all relate to the use of various materials and forms of energy to enhance the regrowth of bone at the interface between an implanted prosthesis and the native bone; however none of them utilize algorithmic control, nor feedback relating to the current status of bone/implant healing, nor do they include means to adapt based on communication external to the body, nor does their active use take into consideraton the various phases of post-surgical rest, initial healing, rehabilitation, normal use, and long term failure monitoring. The disclosure of each of the United States patents described hereinabove is hereby incorporated by reference into this specification.
It is an object of this invention to provide an improved prosthesis implant system which monitors, analyzes, and communicates the status of extent of the healing of bone around an implant and communicates such information to a controller, and, as provide, provides additional therapy to the organism in which the implant is disposed.
In accordance with this invention, there is provided a prosthetic implant system connected to the tissue of a living organism which comprises a first means for measuring the strain in the body of said implant in an area thereof which is not exposed to tissue regrowth, a second means for measuring the strain in the body of said implant in an area thereof which is exposed to tissue regrowth, means for continuously determining the stress/strain relationship of said implant, and means for treating said tissue.